Electric shock defibrillation is a proven technique for treatment of the serious and immediately life-threatening condition of ventricular fibrillation. For patients known to be at risk, an implantable defibrillator may be used. Such devices contain an energy source, an electrode lead system in contact in the heart, a sensing system to detect the onset of fibrillation, and a pulse generator for delivering the defibrillation shock.
The electrode lead system may include electrodes to reverse certain life-threatening arrhythmias, like ventricle fibrillation, where electrical energy is applied to the heart via the electrodes to return the heart to normal rhythm. One type of electrode which may be used is a shock lead. In determining when stimulation is needed, a shock lead system senses activity in the heart through a sensing system, e.g. an electrogram channel. To ensure accurate sensing, the shock lead system's electrogram channel must be properly calibrated to provide a set of values for the heart activity within a preset range.
One method for calibrating a shock lead system is to use a programmer device to determine and set a gain value for the shock electrogram channel. The programmer device, external to the patient, collects real-time electrogram data for a period of time and determines the shock channel gain based on amplitudes obtained while collecting data. This method of gain production is susceptible to errors since real-time electrogram data has no error correction, e.g. CRCs, and can be corrupted by noise signals which are prevalent in hospital rooms, the likely programming environment.
Additionally, it is possible that the shock lead system is not properly calibrated after insertion. This can occur if the device is not properly programmed after insertion or if the device is not calibrated at all. The device may not be calibrated at all if a physician fails to program the device after insertion. If the device is not properly calibrated after insertion it will only have a default gain level that is set prior to implantation of the device. With a default gain level, the output signal from the electrogram may be so low that only a flat line is output and the data is not useful for therapeutic purposes. It is also possible that with bad lead placement the signal coming out would be too large resulting in a clipped signal, again making the data useless therapeutic purposes. In either case, the inserted device will not record electrograms properly until programmed manually by a physician. This change may not occur until a patient returns to the doctor or hospital for examination and, in the meantime, valuable therapeutic data will have been lost.
Thus, what is needed in the art is a method and device for automatically calibrating a shock lead system that ensures signal levels are within a prescribed range for the proper functioning of the implantable medical device.